Persistent modification of synaptic strength is a central in most conceptual models of learning, memory and recovery form injury, yet it has received surprisingly little attention in the CNS of the adult mammal. Our long term objectives are to identify cellular processes involved in persistent alteration in the efficacy of synapses on spinal motoneurons and to relate the findings to changes in motor-unit recruitment behavior. A major aim of this proposal is to determine whether transmission at the synapses made by Ia fibers and alpha motoneurons can be modified by chronically suppressing their activity. Synaptic transmission form Ia fibers will be chronically inhibited by preventing impulses from reaching their synaptic terminals. After 2 weeks of verified impulse blockade we will apply intracellular recording techniques to measure the size of the monosynaptic excitatory postsynaptic potentials (EPSPs) produced in motoneurons by the inactivated Ia fibers. If these studies indicate that inhibition of synaptic usage does alter synaptic efficacy as our preliminary data suggest, then we can begin to assess the underlying mechanism and realize it as a potential means of altering monosynaptic reflex behavior. Alternatively, if no change can be identified, then this synapse would provide a unusual example of one whose efficacy is independent of usage, and through comparison with others that are sensitive to use help to identify the necessary elements for plasticity. Another aim of this proposal is to continue studying the recruitment of motor units in a muscle reinnervated by its own served nerve. Recent studies suggest that changes in synaptic strength are not observed under these conditions and cannot, therefore, account for observations that motor units in reinnervated muscles are not recruited in order by their tension as they are normally. It is instead suggested that recruitment order can be recovered if the muscle is not reinnervated by foreign nerves. Our experiment is designed to test that hypothesis. Using intra-axonal recording and stimulation we will determine the relative recruitment threshold and various electromechanical properties of single motor units in self-reinnervated muscle of anesthetized cats. These data will and in characterizing the performance of reinnervated muscle with respect to fundamental properties such as tension and endurance, and show the extent to which interrelations among motoneurons, their synaptic input and muscle units are collectively restored.